1. Technical Field
The present invention generally relates to a generator brake system and a method of controlling the same, and more particularly to a generator brake system for a permanent magnet generator (PMG) and a method of braking the PMG in a short circuit manner by turning on at least one switch unit.
2. Description of Prior Art
In the current wind power generation system, the wind turbine is provided to convert the wind power into the electric power with a constant frequency by inverters. Hence, the reliability of the wind power generation system as well as that of the inverter system is critical. In addition, the protection ability of the wind power system will be subject to strict tasks because the wind frequently and intensely changes. Also, the inverter system would be damaged when the wind turbine is operated at a high wind speed.
Please refer to FIG. 1 which shows a conventional protection circuit for electrical machines. US 2013/0194704 disclosed the protection circuit and method for electrical machines. The generator is a permanent magnet generator 2 with a three-phase stator winding 4. Stator terminals of the permanent magnet generator 2 are connected to the power converter 6 through the three-phase circuit 8. The three-phase circuit 8 has three separate conductors 10a, 10b, 10c, each of which is connected to a respective phase of the stator winding 4. A contact device 12 includes three separate contacts 14a, 14b, 14c, each of which is connected between one of the conductors 10a, 10b, 10c and a common conductor 16.
When the generator 2 normally operates, the contacts 14a, 14b, 14c are open and electrical power flows from the stator terminals of the generator 2 to the power converter 6 through the three-phase circuit 8. If there is a short circuit or fault in the three-phase circuit 8 or the power converter 6, the generator 2 will start to produce a fault current. At this time, the contacts 14a, 14b, 14c can be closed to connect the conductors 10a, 10b, 10c to the common conductor 16. Once the contacts 14a, 14b, 14c are closed then no fault current will flow from the generator 2 to the power converter 6. Therefore, the addition of the contactor device 12 minimizes any damaged risk of the power converter 6. The contacts 14a, 14b, 14c of the contactor device 12 can be opened again once the fault current has been cleared.
However, once the switch contacts are used to provide brake protection for the generator, the problems of reduction of life time and invalidation of brake protection of the hardware switches would occur because of frequently switching the switches and failure of switch contacts.